1. Field of the Invention
This invention relates in general to a line disconnection detection circuit, and more particularly, to a circuit for comparing transmitted and received data frames containing stuffed symbols to detect when a channel is disconnected.
2. Description of Related Art
Transceivers typically use a hybrid circuit (or balancing circuit, or two-to-four wire converter) to provide first-order cancellation of the transmit signal before the combined receive and transmit signal enters the receiver. Hybrid circuits generally use a simple model of the characteristic impedance of the communication channel in the frequency band of data transmission to create an estimate of the transmit signal component of the channel signal. This estimate is then subtracted from the channel signal to isolate the received signal. The hybrid circuit allows gain to be applied to the signal prior to further processing so that any noise added by the signal processing is negligible with respect to the receive signal level. Further signal processing may involve the step of removing any residual transmit signal component from the channel signal (echo cancellation).
Nevertheless, a problem occurs when an echo cancelling transceiver is passing data in both directions on a communications channel, and that channel is suddenly disrupted. For example, channel disruption may occur when the telephone line is unplugged at the transceiver. Disruption of the hybrid circuit balance results when the channel is disrupted. Accordingly, not only is the transmission link broken and the receive signal eliminated, but a full-sized transmit signal which exceeds the allowable signal voltage range enters the receiver.
Due to the strength of the transmit signal which permeates into the receiver channel, echo cancellers are incapable of cancelling the entire signal. Thus, the uncancelled portion of the transmit signal continues on through the receiver and is decoded as valid data. Accordingly, unless there is some identifier in the data streams to distinguish receive data from transmit data, a channel disconnection will not be detected.
It can seen then that there is a need for a line disconnection detection circuit. Further, there is a need for a line disconnection circuit which can distinguish between the local transmit signal and the remote receive signal.